Coating compositions of this kind are known from WO 08/74491, WO 08/74490, and WO 08/74489, for example. The compound (B) used in these coating compositions and containing isocyanate groups and silane groups is based on known isocyanates, preferably on the biuret dimers and isocyanurate trimers of diisocyanates, more particularly of hexamethylene diisocyanate. These coating compositions of WO 08/074,489 have the advantage over conventional polyurethane coating compositions of significantly enhanced scratch resistance in tandem with good weathering stability. The coating compositions described therein are used more particularly in automotive OEM finishing, although their use in automotive refinish is also described. A disadvantageous aspect to these coating compositions, however, is that a reduction in the solvent fraction, in other words an increase in the nonvolatile fraction, of the coating compositions is associated with a deterioration in the flow properties and hence with a significant impairment of the optical quality of the resultant coatings.
Coating compositions used for the automotive refinish segment, however, are affected by statutory emissions guidelines (e.g., German Federal Airborne Pollutants Ordinance 31). The current European directive prescribes for clearcoat systems a VOC (“volatile organic content”) of 420 g/l, resulting, depending on the density of the system used, in nonvolatile fractions of around 60% by weight. A problematic aspect in the development of systems having a high nonvolatile fraction is that in general there is an increase in viscosity and hence a reduction in the fluidity of the system, with adverse consequences for the flow and topcoat holdout. To counteract this effect, it is necessary to raise the nonvolatile fraction while maintaining the same viscosity. This is generally achieved by reducing the viscosity of the curing agent and/or of the binder. Doing so, however, often entails a deterioration in physical film formation, and results in longer drying times. Especially for coating compositions which are used in the automotive refinish segment, this condition is disadvantageous, since the coating compositions used in the refinish segment offer long reaction times in any case.
U.S. Pat. No. 5,691,439 discloses coating compositions which, in addition to hydroxyl-containing binders (A), comprise compounds (B) with isocyanate groups as crosslinkers, it being essential to the invention that the compounds (B) also have silane groups or siloxane groups, in order to lower the surface energy, and also have allophanate groups, in order to obtain transparent coatings, meaning that coatings are obtained which have improved surface properties. That specification, however, lacks details as to how the solvent fraction of the coating compositions can be lowered while nevertheless ensuring high cure rates even under the conditions of automotive refinish.
Furthermore, EP-A-1 273 640 describes 2K [2-component] coating compositions, comprising a polyol component and a crosslinker component, consisting of aliphatic and/or cycloaliphatic polyisocyanates or the polyisocyanates derived from them by polymerization, allophanatization, biuretization or urethanization, with 0.1 to 95 mol % of the originally free isocyanate groups present having undergone reaction with bisalkoxysilylamine. These coating compositions can be used for producing clearcoats or topcoats in the automotive segment and, when they have fully cured, exhibit high scratch resistance in conjunction with high resistance to environmental influences. That specification, however, lacks details as to how the solvent fraction of the coating compositions can be lowered while still ensuring high cure rates, even under the conditions of automotive refinish, and without detraction from the surface properties of the resultant coatings.
WO 2001/98393 describes 2K [2-component] coating compositions which comprise a polyol as binder component and, as crosslinker component, a polyisocyanate functionalized in low fractions with alkoxysilylamines, preferably with bisalkoxysilylamines. These coating compositions are to employed particularly as primers and are optimized for adhesion to metallic substrates, preferably to aluminum substrates.
Hitherto unpublished international patent application PCT/US 2010/028308 describes coating compositions which in addition to a hydroxyl-containing component (A) and an isocyanate-group-containing component (B) comprise the reaction product of a uretdione with a bisalkoxysilylamine or with a monoalkoxysilylamine, but the reaction production of the uretdione with the alkoxysilylamine no longer contains any residual isocyanate groups.
EP-B-864 575 describes compounds having alkoxysilane groups and urea groups and obtained by reacting polyisocyanates, such as uretdiones and/or isocyanurates, for example, with secondary monoalkoxysilylamines containing ester groups, such as, more particularly, diethyl N-(3-trimethoxysilylpropyl)aspartate. The compounds having alkoxysilane groups and urea groups no longer contain substantially any remaining isocyanate groups, and according to EP-B-864 575 are used, optionally together with further silane-group-containing components, in coating compositions which cure exclusively by way of silane polycondensation. The use of these compounds having alkoxysilane groups and urea groups together with hydroxy-containing components and isocyanate-group-containing components, on the other hand, is not described in EP-B-864 575.
Lastly, EP-A-1 426 393 discloses polyisocyanates which contain uretdione groups, are of low monomer content, and have the advantage, moreover, that their stability with respect to retrograde cleavage is improved over that of the uretdiones formerly used. This improved stability with respect to retrograde cleavage is acquired by dimerizing the uretdiones at temperatures of <=40° C. in the presence of trialkylphosphines and then separating off the trialkylphosphines. The uretdiones are used, for example, as curing agents in coating compositions. That specification, however, lacks details of how the surface properties of the resultant coatings can be influenced, and details as to how effective curing can be ensured even under the conditions of refinish.
The problem addressed by the present invention, therefore, was that of providing coating compositions, more particularly for automotive refinish, which ensure effective curing even under refinish conditions, have a high solids content and hence a very low solvent content, exhibit good flow and topcoat holdout, and lead to coatings having good surface properties.
The overall optical appearance was assessed by measuring the surface profile of the applied and baked coating films, using the wavescan method, which allows measurement of the visible profile of coating film surfaces. For this purpose, the intensity of reflection (“waviness”) was measured by means of the Byk-Gardner Wave Scan instrument, recording 1250 measurement points over a distance of 10 cm. The instrument divides the reflection into longwavedness (“long-wave”), i.e., the variance in light intensity for structures in the range from 0.6 mm to 10 mm, and into shortwavedness (“short-wave”), i.e., the variance in light intensity for structures in the range from 0.1 mm to 0.6 mm. For a good appearance, low long-wave measurement values in the resultant coatings, for very low film thicknesses, are particularly critical.
Furthermore, the intention was to provide coating compositions which lead to a highly weathering-stable network and which at the same time ensure high acid resistance. In addition, the intention was that the coating compositions should lead to coatings which are highly scratch-resistant and more particularly exhibit a high level of gloss retention after scratch exposure. Moreover, the coatings and paint finishes, especially the clearcoats, ought to be able to be produced even in film thicknesses >40 μm without stress cracks occurring. Furthermore, the coating compositions ought to meet the requirements typically imposed on the clearcoat film in automotive OEM finishes and automotive refinishes.
Finally, the new coating compositions ought to be producible easily and with very good reproducibility, and ought not to cause any environmental problems in the course of paint application.